In a turbocharger, surging is a phenomenon that, when a turbocharger is operated in a state where the intake flow rate of the compressor on the intake-system side is low, the operation state of the intake compression mechanism inside the compressor vibrates, and the intake flow rate, the intake pressure and the rotation speed fluctuate, where the turbocharger may become incapable of operating in a serious case. The operation characteristics of a turbocharger can be evaluated by using a pressure curve which represents a relationship between the pressure ratio and the intake flow rate of a compressor. In this case, the surging region in which surging of a turbocharger occurs exists in a region with a relatively low intake flow rate compared to a pressure ratio.
When the operation state of a turbocharger enters the turbocharger, abnormal sounds occur due to fluctuation of the boost pressure of the pressure wave vibration, where the turbocharger may become damaged from vibration of the operation state of the compressor in a serious case. Thus, to avoid or prevent damage of the like of a turbocharger, it is necessary to detect occurrence of surging or its predictor at an earliest possible state, and to perform operation for surging avoidance. As described above, surging of a turbocharger can be detected on the basis of a relationship between the pressure ratio and the intake flow rate of a compressor. Thus, surging can be detected by monitoring measurement values of the pressure ratio and the intake flow rate of the turbocharger with a pressure sensor and an intake flow rate sensor provided additionally for the turbocharger. However, additionally providing a pressure sensor and an intake flow rate for a turbocharger may lead to an increase in the costs and work hours for manufacturing turbochargers, and thus is not desirable.
Patent Document 1 discloses a surging detection mechanism for detecting surging by using only a rotation-speed sensor normally provided for a common turbocharger for measuring the rotation speed of the turbocharger turbine shaft, without additionally providing a pressure sensor and an intake flow rate sensor for the turbocharger. The surging detection mechanism described in Patent Document 1 calculates a rotation speed suitable as the rotation speed of a turbocharger on the basis of the current engine rotation speed and the current engine load, and determines that surging is occurring if the measurement value of the current rotation speed of the turbocharger is not smaller than the above suitable rotation speed.